Compared to earlier systems, modern airborne systems push the limits of technology to achieve greater speed, payload, and efficiency. To accomplish these objectives, advanced aircraft structural designs often employ unitized bonded layered structures (e.g., resins or plastic reinforced with fibers) while former methods required that a multitude of components be joined (e.g., bolted, riveted, and/or welded) with a plurality of access points (i.e., inspection points).
In aviation, weight is a crucial factor, and removing or reducing the number of access points within a structure reduces the overall aircraft weight. While unitized bonded layered structures reduce weight, they also reduce the number of access points, creating the necessity for new maintenance and inspection strategies to compensate for the reduced number of access points. For example, as advanced layered structures become more unitized, inspection of remote and inaccessible locations for any damage caused by Foreign Object Debris (FOD) or liquid intrusion becomes increasingly difficult, sometimes impossible, while at the same time making inspection and maintenance a higher priority. Current inspection methods typically involve physically removing one or more unitized aircraft structural components to enable proper inspections and ensure that critical structures have not been damaged. However, not surprisingly, this endeavor is costly in terms of both manpower and loss of vehicle availability.
For example, in the case of the Sikorsky CH-53E Super Stallion, a heavy-lift cargo helicopter in service with the U.S. Marine Corps, up to 30-40 man-hours per flight hour may be necessary to ensure the helicopter is structurally sound. Currently, non-destructive evaluation and inspection (NDE/NDI) methods for detecting damage or liquid intrusion must be completed by touch labor (manual inspection) on and/or off the aircraft. Unfortunately, these in-depth, labor-intensive inspections also have the consequence of providing further opportunities for damage through accidents such as tool drops or FOD.
Furthermore, it is highly desirable to be able to characterize and identify material aging issues such as matrix cracking, delaminating, and water intrusion into layered panels without the need to perform detailed NDE inspections or tear-down tests that require large amounts of time, highly skilled labor, and removal of structure. More recently, aging is becoming a real concern for layered structures as early layered components are reaching service life times where the effects of aging have not yet been quantified and the remaining useful life is unknown. For instance, tear-down tests of graphite/epoxy components by the National Institute For Aviation Research (NIAR) on the Boeing 737-200 and Beechcraft Starship have shown little degradation of the structures over their respective 18- and 12-year histories. However, exposure to ultraviolet light and moisture can break down the fiber matrix bond over time, while micro-cracking can lead to stress risers at the crack tip, reducing service life. These types of issues are often precursors to failure, and detecting them prior to failure can be quite difficult. Not surprisingly, layered aerial vehicles also age in relation to overall system maintenance and inspection routines, where the probability of degraded structure increases as events that may compromise the pristine structure occur and remain unrepaired.
Thus, what is needed is an onboard Structural Health Monitoring (SHM) system enabled to perform maintenance inspections with a high probability of detection without requiring physical or visual access to the components, thereby decreasing the cost of maintenance on current and future air vehicles.